Geological sciences geology landforms, minerals, and rocks

The Division of Earth Sciences 2Knowledge of Earth Knowledge of Landforms Earth History According to... Each one considers different facets of Earth’s surface and inte-rior, such as i

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On the cover (front and back): Geological layers, grave, Petra, Jordan Patrice Hauser/

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On the cover (front top), p 1: Stalagtites and stalagmites in an illuminated cave (far left);

paleontologists chip at a rock face in Madagascar (second from left); geologic folds at a cliff face in Spain (second from right); a geologist works with a pickaxe to dislodge samples

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The Division of Earth Sciences 2

Knowledge of Earth

Knowledge of Landforms

Earth History According to

The Chemical Analysis of

Electrical and Electromagnetic

Activities in the United States 132

122

I ntroductIon

IntroductIon

7 Introduction 7

The Earth sciences are a collection of disciplines that

consider Earth’s atmosphere and hydrosphere, as well

as the planet’s solid aspect, the geosphere The academic disciplines concerned with the geosphere are collectively called the geological sciences, or geosciences Each one considers different facets of Earth’s surface and inte-rior, such as its rocks, minerals, and their chemistry, the evolution and role of its landforms, and its geologic his-tory Although most of the geosciences exist to develop a greater understanding of the parts and processes involved

in the solid Earth, the subfield of economic geology takes

as its mission the extraction of rocks and minerals and their conversion to useful products

This book initiates readers into the study of geology and the rest of the geological sciences Along the way, readers will meet many of the explorers and thinkers that plumbed the geosphere and laid the foundations of geologic study, learning about their contributions and vir-tually examining some of the tools they used to draw their conclusions

The work of modern geoscientists is the direct result

of knowledge gained from thousands of years of tion and investigation Initial forays into the geological sciences, which likely occurred before written records were kept, probably involved the collection of useful stones and gems, as well as observances of earthquakes and volcanic activity The ancient Greeks and the Chinese were the first to record geological phenomena, seeing fos-sils as forms of ancient life and clues to environments of the past rather than as simple curiosities

observa-Major advances in the study of geology, however, did not occur until the 1500s and later During this period, the

Geologists at work in Chile’s La Escondida mine Keith Wood/The Image

Bank/Getty Images

basic tenets of stratigraphy, which is the study and sification of rock layers, were put forth In the late 1660s, Danish scientist and theologian Nicolaus Steno developed the principle of superposition, which states that younger layers of rock rest above older layers.

clas-Other developments followed Scottish scientist James Hutton described the concept of uniformitarian-ism, which maintains that the geologic processes that take place in the present also occurred in the past Thus, past geologic events, such as the changes in ancient river basins, could be explained by processes that were still occurring today In the early 19th century, the work of French scien-tist René-Just Häuy concerning minerals and their crystal features produced the science of crystallography In 1837 Louis Agassiz, a Swiss-born scientist and teacher from the United States, posited that the placement of large boulders far from their points of origin resulted from the movements of tremendous ice sheets

In 1905, the first steps toward developing radiometric dating, a technique designed to calculate the approximate age of a rock or mineral, were made by American chemist Bertram Boltwood Noting that the shape of the western coast of Africa could theoretically fit together with the eastern coast of South America, German meteorologist Alfred Wegener proposed the theory of continental drift

in 1912 Wegener’s theory held that continents were not stationary, but, rather, that they moved to new positions across vast intervals of geologic time

A watershed moment in the development of the logical sciences occurred in the 1960s Scientists from the United States and the United Kingdom uncovered evidence that new oceanic crust formed along the mid-oceanic ridges, a long chain of underwater mountains that occur at the boundaries between Earth’s tectonic plates,

geo-and that these ridges were spreading Rocks in the new crust also recorded periodic reversals of Earth’s magnetic field This new information enabled scientists to develop

a driving mechanism for Wegener’s theory and better explain the dynamics of several other geologic processes, such as volcanic eruptions and rock folding

Today, the jumping-off point for the modern study of the geologic sciences is geology, which is the discipline concerned with Earth, the materials that form it, and the various chemical reactions and physical forces that act upon the planet’s surface and its interior At the heart of geology is mineralogy, the subdiscipline that focuses on the classification of minerals and the study of their charac-teristics and behavior Minerals are the basic components

of rocks They are naturally occurring solids containing unique crystalline geometries that reflect their unique chemical structures The study of a mineral’s geometric properties and internal structure fall within the purview

of crystallography, whereas the study of its chemical ture, as well as that of the rock that contains it, falls to the subdiscipline of geochemistry

struc-Mineralogists, petrologists (scientists who study rocks), crystallographers, and geochemists are only a fraction of the people working within the geosciences Geodesists, geophysicists, and structural geologists con-sider Earth’s structure beyond the scale of individual rocks and minerals The science of geodesy investigates Earth’s size and shape and provides the means, through a series of surveyed points on the surface, to create maps of Earth’s features Such maps and reference points may be used by other geoscientists to frame their own investigations Geophysics is a wide-ranging discipline concerned with changes to Earth’s gravitational field, the movement

of seismic waves and electricity through Earth’s crust and

interior, and the role Earth’s magnetic field plays on the planet’s geology This science also considers how Earth’s magnetic field behaves when it is exposed to different types of external radiation, the transmission of heat from the planet’s interior, and how all of these factors interact with one another

Similarly, structural geology covers a wide area This subdiscipline spans everything from the imperfections within a given mineral crystal to the forces that shape mountains and Earth’s tectonic plates Another subdisci-pline, tectonics, strives to make comprehensible how the planet formed and how it continues to evolve, whereas volcanology seeks to understand the behaviour of volca-noes and their contributions to Earth’s crust

Other subdisciplines of geology focus exclusively on Earth’s surface features and the forces that alter them Geomorphology attempts to understand the processes that create and destroy landforms For example, fluvial geomorphologists—that is, those that examine the forces and processes that occur in river systems—study how the movement of water affects the various landforms that occur within watersheds, as well as those landscape fea-tures (river banks, streambeds) that appear within the river itself Water is also the focus of glacial geology, but only when this ubiquitous compound occurs as ice This particular subdivision examines the behaviour of ice and how its movement can create, destroy, and modify Earth’s surface features

Geologists also seek to establish a timeline of major events in Earth’s history, such as the movements of conti-nents, the evolution of life, the colonization of the land by trees, and the timing of mass extinctions Earth’s history, which spans approximately 4.5 billion years, is far longer than recorded human history, so geologists look for clues

in rocks One of the aforementioned principles of geology states that, in general, the layers of sedimentary rock get older the deeper one digs The order of the rock layers can provide the geologist with a sense of the sequencing of spe-cific geologic events In addition, the absolute age of some rocks can be determined by examining the decay of the radioactive isotopes contained therein

Other clues can be found in the fossilized remains of certain organisms Some fossils, called index fossils, have tremendously large distributions that span multiple con-tinents They can be used to assist in understanding the relative age of a rock, the environmental conditions pres-ent when the rock was formed, and the orientation of the different landmasses upon which the rock was discovered The examination of fossils falls within the purview

of paleontology Paleontologists are scientists who study extinct life The field is divided up into three parts: inver-tebrate paleontology, which generally focuses on fossil invertebrates from marine environments; vertebrate paleontology, which examines fossil animals with back-bones; and micropaleontology, which investigates fossil zooplankton, such as tiny crustaceans and foraminifera Fossil plants, which include different types of algae, also are helpful in establishing the timeline of geologic events Paleobotany is the field concerned with their study, whereas the study of pollen, spores, and very tiny plank-tonic organisms is considered within the broad field of palynology

Geology is not necessarily restricted to Earth Other planets and solid bodies in the solar system and beyond are composed of rock The geology of some of these worlds may be affected by the same forces that appear on Earth,

or they may be the product of utterly alien conditions, for example, their proximity to the local star or their orbit

The study of the geology in worlds beyond our own is called astrogeology.

One of the most important subdisciplines of ogy, in that it affects the lives of most human beings on a daily basis, is economic geology Modern civilization can-not function without materials extracted from the solid Earth The development of techniques to find and recover petroleum from between deep layers of rock is probably one of this field’s most important activities Fuel oil for heating and gasoline and diesel fuel for transportation are some of the world’s most valuable products Along with coal and natural gas, petroleum-derived fuels—often referred to as fossil fuels keep automobiles and other vehicles moving and electricity flowing Without these services, the economies of many societies would grind to

geol-a hgeol-alt Thousgeol-ands of other products, in geol-addition to fuels, are made from petroleum Plastics, synthetic rubbers and fabrics, cosmetics, road tar, and waxes are all made from this material Petroleum derivatives are even used in foods and medicines

The fruits of economic geology also extend to other industries In many areas around the world, soils must be stabilized with sand, gravel, and rock to prevent the col-lapse or degradation of buildings that are constructed upon them Additionally, many of the materials used to build houses and other structures are extracted from the ground Limestone and clay are ingredients in cement used to create a structure’s foundation, sheets of drywall made of gypsum often separate interior spaces, and copper

is used to make electrical wiring Other metals recovered from the solid Earth are used to make nails, screws, rein-forcing bars in concrete (rebar), joints, pipes, and parts of the ventilation system In addition, the extraction of pre-cious minerals (such as diamonds, rubies, and sapphires

from corundum, and emeralds from beryl) and precious ores such as gold, silver, and platinum supports more than just the jewelry industry Many of these materials are used

to build tools for industrial processes or parts for tronic devices

elec-The geological sciences make up an important aspect

of the Earth sciences They are a collection of disciplines that contribute greatly to the understanding of the mate-rials that make up the solid Earth and the structure of the planet’s interior Some of these disciplines—most nota-bly geochemistry, geophysics, and paleontology—employ the tools and techniques of the other sciences in order

to discover and explain geological phenomena As new technologies with which to study the solid Earth emerge, teams of different specialists from the geosciences and other fields will continue to collaborate with one another

to unlock the mysteries of the planet

CHAPTER 11

The Earth sciences are made up of the fields of study

concerned with the solid Earth, its waters, and the air that envelops it The broad aim of the Earth sciences is

to understand the present features and the past evolution

of Earth and to use this knowledge, where appropriate, for the benefit of humankind Thus the basic concerns of the Earth scientist are to observe, describe, and classify all the features of Earth, whether characteristic or not, in order

to generate hypotheses with which to explain their ence and development Earth scientists also devise means

pres-of checking opposing ideas for their relative validity In this way the most plausible, acceptable, and long-lasting ideas are developed

The geologic sciences constitute one division of the Earth sciences Geology and its related subfields focus on the phenomena occurring within the planet or on its sur-

the phenomena occurring within the planet or on its surface The Earth sciences also include the hydrologic and atmospheric sciences

It is worth emphasizing two important features that the geological sciences have in common with the other two divisions of the Earth sciences First is the inacces-sibility of many of the objects of study Many rocks, as well

as water and oil reservoirs, are at great depths in Earth, while air masses circulate at vast heights above it Second, there is the fourth dimension—time Geological scientists are responsible for working out how Earth evolved over millions of years For example, what were the physical and

THE DIVISION OF

EARTH SCIENCES

Today the Earth sciences are divided into many disciplines, which are themselves divisible into six groups Although a few of the disciplines listed below fall within the scope of the hydrologic and atmospheric sciences, the majority relate directly to the science of geology and its related subdisciplines.

1 Those subjects that deal with the water and air at or above the solid surface of Earth These include the study of the water on and within the ground (hydrology), glaciers and ice caps (glaciology), oceans (oceanography), the atmosphere and its phenomena (meteorology), and world climates (clima- tology) Such fields of study are grouped under the hydrologic and atmospheric sciences and are treated separately from the geologic sciences, which focus on the solid Earth.

2 Disciplines concerned with the physical-chemical makeup of the solid Earth, which include the study of minerals (mineral- ogy), the three main groups of rocks (igneous, sedimentary, and metamorphic petrology), the chemistry of rocks (geo- chemistry), the structures in rocks (structural geology), and the physical properties of rocks on Earth’s surface and within its interior (geophysics).

3 The study of landforms (geomorphology), which is cerned with the description of the features of the present terrestrial surface and an analysis of the processes that gave rise to them.

con-4 Disciplines concerned with Earth’s geologic history, ing the study of fossils and the fossil record (paleontology), the development of sedimentary strata deposited typically over millions of years (stratigraphy), and the isotopic chemis- try and age dating of rocks (geochronology).

includ-5 Applied Earth sciences dealing with current practical cations beneficial to society These include the study of fossil fuels (oil, natural gas, and coal); oil reservoirs; mineral deposits; geothermal energy for electricity and heating; the structure and composition of bedrock for the location of

appli-7 Evolution of the Geologic Sciences 7

bridges, nuclear reactors, roads, dams, and skyscrapers and other buildings; hazards involving rock and mud avalanches, volcanic eruptions, earthquakes, and the collapse of tunnels; and coastal, cliff, and soil erosion.

6 The study of the rock record on the Moon, the planets, and their satellites (astrogeology) This field includes the investigation of relevant terrestrial features—namely, tek- tites (glassy objects resulting from meteorite impacts) and astroblemes (meteorite craters).

With such intergradational boundaries between the divisions

of the Earth sciences—which, on a broader scale, also overlap with physics, chemistry, biology, mathematics, and certain branches of engineering—researchers today must be versatile in their approach to problems.

chemical conditions operating on Earth and the Moon 3.5 billion years ago? How did the oceans and atmosphere form, and how did their chemical composition change with time? How did life begin, and how has life evolved?

ORIGINS IN PREHISTORIC TIMES

The origins of the geological sciences lie in the myths and legends of the distant past The creation story, which can

be traced to a Babylonian epic of the 22nd century BCE

and is told in the first chapter of Genesis in the bible, has proved most influential The story is cast in the form of Earth history and thus was readily accepted as an embodi-ment of scientific as well as of theological truth

Earth scientists later made innumerable observations

of natural phenomena and interpreted them in an ingly multidisciplinary manner The geological and other

increas-Earth sciences, however, were slow to develop largely because the progress of science was constrained by what-ever society would tolerate or support at any one time.

ANTIqUITy

Humans likely studied Earth’s structure, composition, and geologic history since before the dawn of writing The Greeks and the Chinese were among the first peoples to record their observations Despite limited technology, geological scientists of the time made the first attempts to classify and describe different planetary phenomena

Knowledge of Earth composition

and Structure

The oldest known treatise on rocks and minerals is the

De lapidibus (“On Stones”) of the Greek philosopher

Theophrastus(c 372–c 287 bCe) Written probably in the early years of the 3rd century, this work remained the best study of mineral substances for almost 2,000 years Although reference is made to some 70 different mate-rials, the work is more an effort at classification than systematic description

In early Chinese writings on mineralogy, stones and rocks were distinguished from metals and alloys, and fur-ther distinctions were made on the basis of colour and other physical properties The speculations of Zheng Sixiao (died 1332 Ce) on the origin of ore deposits were more advanced than those of his contemporaries in Europe In brief, his theory was that ore is deposited from groundwater circulating in subsurface fissures

Ancient accounts of earthquakes and volcanic tions are sometimes valuable as historical records but tell little about the causes of these events Aristotle (384–322

erup-bCe) and Strabo (64 bCe–c 21 Ce) held that volcanic sions and earthquakes alike are caused by spasmodic motions of hot winds that move underground and occa-sionally burst forth in volcanic activity attended by Earth tremors Classical and medieval ideas on earthquakes and volcanoes were brought together in William Caxton’s

explo-Mirrour of the World (1480) Earthquakes are here again

related to movements of subterranean fluids Streams of water within Earth compress the air in hidden caverns If the roofs of the caverns are weak, they rupture, causing cities and castles to fall into the chasms; if strong, they merely tremble and shake from the heaving by the wind below Volcanic action follows if the outburst of wind and water from the depths is accompanied by fire and brim-stone from hell

The Chinese have the distinction of keeping the most faithful records of earthquakes and of inventing the first instrument capable of detecting them Records of the dates on which major quakes rocked China date to 780

bCe To detect quakes at a distance, the mathematician, astronomer, and geographer Zhang Heng (78–139 Ce) invented what has been called the first seismograph

Knowledge of Earth History

The occurrence of seashells embedded in the hard rocks

of high mountains aroused the curiosity of early naturalists and eventually set off a controversy on the origin of fos-sils that continued through the 17th century Xenophanes

of Colophon (flourished c 560 bCe) was credited by later writers with observing that seashells occur “in the midst

of earth and in mountains.” He is said to have believed that these relics originated during a catastrophic event that caused earth to be mixed with the sea and then to settle, burying organisms in the drying mud For these

views Xenophanes is sometimes called the father of paleontology.

Petrified wood was described by Chinese scholars as early as the 9th century CE and, about 1080, Shen Gua cited fossilized plants as evidence for change in climate Other kinds of fossils that attracted the attention of early Chinese writers include spiriferoid brachiopods (“stone swallows”), cephalopods, crabs, and the bones and teeth of reptiles, birds, and mammals Although these objects were commonly collected simply as curiosities or for medicinal

Fossilized leaf PhotoObjects.net/Jupiterimages

A fossil is a remnant, impression, or trace of an animal or plant of a past geologic age that has been preserved in Earth’s crust The complex of data recorded in fossils worldwide—known as the fossil record—is the primary source of information about the history of life on Earth.

Fossilized footprint of an unidentified dinosaur © Getty Images

Only a small fraction of ancient organisms are preserved as sils Usually only organisms that have a solid and resistant skeleton (vertebrates) are readily preserved Most major groups of invertebrate animals have a calcareous skeleton or shell (e.g., corals, mollusks, bra- chiopods) Other forms have shells of calcium phosphate (which also occurs in the bones of vertebrates) or silicon dioxide A shell or bone that is buried quickly after deposition may retain these organic tis- sues, though they become petrified (converted to a stony substance) over time Unaltered hard parts, such as the shells of clams or bra- chiopods, are relatively common in sedimentary rocks, and some are quite old.

fos-The hard parts of organisms that become buried in sediment may be subject to a variety of other changes during their conversion

to solid rock, however Solutions may fill the interstices, or pores, of the shell or bone with calcium carbonate or other mineral salts and thus fossilize the remains, in a process known as permineralization

In other cases there may be a total replacement of the original skeletal material by other mineral matter, a process known as mineralization,

or replacement In still other cases, circulating acid solutions may dissolve the original shell but leave a cavity corresponding to it, and circulating solutions of calcium carbonate or silica may then deposit

a new matrix in the cavity, thus creating a new impression of the original shell.

By contrast, the soft parts of animals or plants are very rarely served The embedding of insects in amber and the preservation of the carcasses of Pleistocene-era mammoths in ice are rare but striking examples of the fossil preservation of soft tissues Traces of organisms may also occur as tracks or trails or even borings.

pre-The study of the fossil record has provided important tion for at least four different purposes The progressive changes observed within an animal group are used to describe the evolution

informa-of that group Fossils also provide the geologist a quick and easy way

of assigning a relative age to the strata in which they occur The sion with which this may be done in any particular case depends on the nature and abundance of the animal, since some fossil groups were deposited during much longer time intervals than others Fossils used

preci-to identify geologic relationships are known as index fossils.

purposes, Shen Gua recognized marine invertebrate sils for what they are and for what they imply historically Observing seashells in strata of the Taihang Mountains, he concluded that this region, though now far from the sea, must once have been a shore

fos-Knowledge of Landforms and

of Land-Sea Relations

Changes in the landscape and in the position of land and sea related to erosion and deposition by streams were recognized by some early writers The Greek historian

Herodotus (c.

Herodotus ( 484–c 484–c. 484– 426 BCE E E) correctly concluded that ) correctly concluded that

the northward bulge of Egypt into the Mediterranean is caused by the deposition of mud carried by the Nile.The early Chinese writers were not outdone by the Romans and Greeks in their appreciation of changes

wrought by erosion In the Jinshu (“History of the Jin

Dynasty”), it is said of Du Yu (222–284 Ce) that when he ordered monumental stelae to be carved with the records

of his successes, he had one buried at the foot of a tain and the other erected on top He predicted that in time they would likely change their relative positions, because the high hills will become valleys and the deep valleys will become hills

moun-Aristotle guessed that changes in the position of land and sea might be cyclical in character, thus reflecting some sort of natural order If the rivers of a moist region should build deltas at their mouths, he reasoned, seawater would

be displaced and the level of the sea would rise to cover some adjacent dry region A reversal of climatic conditions might cause the sea to return to the area from which it had previously been displaced and retreat from the area previ-ously inundated The idea of a cyclical interchange between

land and sea was elaborated in the Discourses of the Brothers

of Purity, a classic Arabic work written between 941 and

982 Ce by an anonymous group of scholars at Basra (Iraq) The rocks of the lands disintegrate and rivers carry their wastage to the sea, where waves and currents spread it over the seafloor There the layers of sediment accumulate one above the other, harden, and, in the course of time, rise from the bottom of the sea to form new continents Then the process of disintegration and leveling begins again

was collected, and no new ideas were produced out the Middle Ages, much of which was appropriately called the Dark Ages It was not until the Renaissance in the early 16th century that the geological sciences began

through-to develop again

ore deposits and Mineralogy

A common belief among alchemists of the 16th and 17th centuries held that metalliferous deposits were gener-ated by heat emanating from Earth’s centre but activated

by the heavenly bodies

The German scientist Georgius Agricola has with much justification been called the father of mineralogy

Of his seven geologic books, De natura fossilium (1546;

“On Natural Fossils”) contains his major contributions to mineralogy and, in fact, has been called the first textbook

on that subject In Agricola’s time and well into the 19th century, “fossil” was a term that could be applied to any object dug from the Earth Thus Agricola’s classification

of fossils provided pigeonholes for organic remains, such

as ammonites, and for rocks of various kinds in addition to minerals Individual kinds of minerals, their associations and manners of occurrence, are described in detail, many for the first time

With the birth of analytical chemistry toward the ter part of the 18th century, the classification of minerals

lat-on the basis of their compositilat-on at last became possible The German geologist Abraham Gottlob Werner was one

of those who favoured a chemical classification in ence to a “natural history” classification based on external appearances His list of several classifications, published posthumously, recognized 317 different substances ordered

prefer-in four classes

Paleontology and Stratigraphy

During the 17th century the guiding principles of tology and historical geology began to emerge in the work

paleon-of a few individuals Nicolaus Steno, a Danish scientist and theologian, presented carefully reasoned arguments favouring the organic origin of what are now called fossils Also, he elucidated three principles that made possible the reconstruction of certain kinds of geologic events in

a chronological order In his Canis carcariae dissectum caput

(1667; “Dissected Head of a Dog Shark”), he concluded that large tongue-shaped objects found in the strata of Malta were the teeth of sharks, whose remains were bur-ied beneath the seafloor and later raised out of the water

to their present sites

This excursion into paleontology led Steno to front a broader question How can one solid body, such as a shark’s tooth, become embedded in another solid body, such as a layer of rock? He published his answers in 1669 in a paper titled “De solido intra natu-raliter contento dissertationis” (“A Preliminary Discourse Concerning a Solid Body Enclosed by Processes of Nature Within a Solid”) Steno cited evidence to show that when the hard parts of an organism are covered with sedi-ment, it is they and not the aggregates of sediment that are firm Consolidation of the sediment into rock may come later, and, if so, the original solid fossil becomes encased in solid rock He recognized that sediments settle from fluids layer by layer to form strata that are origi-nally continuous and nearly horizontal His principle

con-of superposition con-of strata states that in a sequence con-of strata, as originally laid down, any stratum is younger than the one on which it rests and older than the one that rests upon it

In 1667 and 1668 the English physicist Robert Hooke read papers before the Royal Society in which he expressed many of the ideas contained in Steno’s works Hooke argued for the organic nature of fossils Elevation of beds containing marine fossils to mountainous heights he attributed to the work of earthquakes Streams attacking these elevated tracts wear down the hills, fill depressions with sediment, and thus level out irregularities of the landscape.

Earth History According to

Werner and James Hutton

The two major theories of the 18th century were the Neptunian and the Plutonian The Neptunists, led by Abraham Gottlob Werner and his students, maintained that Earth was originally covered by a turbid ocean The first sediments deposited over the irregular floor of this universal ocean formed the granite and other crystalline rocks Then as the ocean began to subside, “stratified” rocks were laid down in succession The “volcanic” rocks were the youngest; Neptunists took small account of vol-canism and thought that lava was formed by the burning

of coal deposits underground

The Scottish scientist James Hutton, leader of the Plutonists, viewed Earth as a dynamic body that functions

as a heat machine Streams wear down the continents and deposit their waste in the sea Subterranean heat causes the outer part of Earth to expand in places, uplifting the compacted marine sediments to form new continents Hutton recognized that granite is an intrusive igneous rock and not a primitive sediment as the Neptunists claimed Intrusive sills and dikes of igneous rock provide evidence for the driving force of subterranean heat Hutton viewed

Catastrophism is the doctrine that explains the differences in fossil forms encountered in successive stratigraphic levels as being the prod- uct of repeated cataclysmic occurrences and repeated new creations This doctrine generally is associated with the great French naturalist Baron Georges Cuvier (1769–1832) One 20th-century expansion on Cuvier’s views, in effect, a neocatastrophic school, attempts to explain geologic history as a sequence of rhythms or pulsations of mountain building, transgression and regression of the seas, and evolution and extinction of living organisms.

Uniformitarianism, however, differs significantly from phism Uniformitarianism is the doctrine that existing processes acting in the same manner and with essentially the same intensity as

catastro-at present are sufficient to account for all geologic change It posits that natural agents now at work on and within Earth have operated with general uniformity through immensely long periods of time When William Whewell, a University of Cambridge scholar, intro- duced the term in 1832, the prevailing view (called catastrophism) was that Earth had originated through supernatural means and had been affected by a series of catastrophic events such as the biblical Flood In contrast to the catastrophic view of geology, the principle of uniformity postulates that phenomena displayed in the rocks may be entirely accounted for by geologic processes that continue to operate

CATASTROPHISM AND UNIFORMITARIANISM

great angular unconformities separating sedimentary sequences as evidence for past cycles of sedimentation,

uplift, and erosion His Theory of the Earth, published as an

essay in 1788, was expanded to a two-volume work in 1795 John Playfair, a professor of natural philosophy, defended Hutton against the counterattacks of the Neptunists, and

his Illustrations of the Huttonian Theory (1802) is the clearest

contemporary account of Plutonist theory

at the present day—in other words, the present is the key to the past This principle is fundamental to geologic thinking and underlies the

whole development of the science of geology The term

uniformitari-anism, however, has passed into history, for the controversy between

catastrophists and uniformitarians has largely died Geology as an applied science draws on the other sciences, but in the early 19th cen- tury geologic discovery had outrun the physics and chemistry of the day As geologic phenomena became explicable in terms of advancing physics, chemistry, and biology, the reality of the principle of unifor- mity as a major philosophical tenet of geology became established and the controversy ended.

THE 19TH CENTURY

The 19th century was a period of rapid development in the geologic sciences The first forays into crystallography were made during this time In addition, the 19th century saw the rise of uniformitarianism, the development of the principal of faunal succession, and attempts to describe geologic time

Crystallography and the Classification

of Minerals and Rocks

The French scientist René-Just Häuy, whose treatises on mineralogy and crystallography appeared in 1801 and 1822, respectively, has been credited with advancing mineralogy

to the status of a science and with establishing the ence of crystallography From his studies of the geometric relationships between planes of cleavage, he concluded that the ultimate particles forming a given species of min-eral have the same shape and that variations in crystal habit reflect differences in the ways identical molecules are put together In 1814 Jöns Jacob Berzelius of Sweden

sci-published a system of mineralogy offering a sive classification of minerals based on their chemistry Berzelius recognized silica as an acid and introduced into mineralogy the group known as silicates At mid-century

comprehen-the American geologist James Dwight Dana’s System of

Mineralogy, in its third edition, was reorganized around a

chemical classification, which thereafter became standard for handbooks

The development of the polarizing microscope and the technique for grinding sections of rocks so thin as

to be virtually transparent came in 1827 from studies of fossilized wood by William Nicol In 1849 Clifton Sorby showed that minerals viewed in thin section could be identified by their optical properties, and soon afterward improved classifications of rocks were made on the basis

of their mineralogic composition The German

geolo-gist Ferdinand Zirkel’s Mikroscopische Beschaffenheit der

Mineralien und Gesteine (1873; “The Microscopic Nature of

Minerals and Rocks”) contains one of the first mineralogic classifications of rocks and marks the emergence of micro-scopic petrography as an established branch of science

William Smith and Faunal Succession

In 1683 the zoologist Martin Lister proposed to the Royal Society that a new sort of map be drawn showing the areal distribution of the different kinds of British “soiles” (veg-etable soils and underlying bedrock) The work proposed

by Lister was not accomplished until 132 years later, when

William Smith published his Geologic Map of England and

Wales with Part of Scotland (1815).

A self-educated surveyor and engineer, Smith had the habit of collecting fossils and making careful note of the strata that contained them He discovered that the differ-ent stratified formations in England contain distinctive

assemblages of fossils His map, reproduced on a scale of five miles to the inch, showed 20 different rock units, to which Smith applied local names in common use—e.g., London Clay and Purbeck Beds In 1816 Smith published

a companion work, Strata Identified by organized Fossils,

in which the organic remains characteristic of each of his rock units were illustrated His generalization that each formation is “possessed of properties peculiar to itself [and] has the same organized fossils throughout its course” is the first clear statement of the principle of fau-nal sequence, which is the basis for worldwide correlation

of fossiliferous strata into a coherent system Smith thus demonstrated two kinds of order in nature: order in the spatial arrangement of rock units and order in the succes-sion of ancient forms of life

Smith’s principle of faunal sequence was another way

of saying that there are discontinuities in the sequences of fossilized plants and animals These discontinuities were interpreted as indicators of episodic destruction of life or

as evidence for the incompleteness of the fossil record Baron Georges Cuvier of France was one of the more dis-tinguished members of a large group of naturalists who believed that paleontological discontinuities bore witness

to sudden and widespread catastrophes Cuvier’s skill at comparative anatomy enabled him to reconstruct from fragmentary remains the skeletons of large vertebrate ani-mals found at different levels in the Cenozoic sequence

of northern France From these studies he discovered that the fossils in all but the youngest deposits belong to spe-cies now extinct Moreover, these extinct species have definite ranges up and down in the stratigraphic column Cuvier inferred that the successive extinctions were the result of convulsions that caused the strata of the conti-nents to be dislocated and folded and the seas to sweep across the continents and just as suddenly subside

Charles Lyell and Uniformitarianism

In opposition to the catastrophist school of thought, the British geologist Charles Lyell proposed a uniformi-

tarian interpretation of geologic history in his Principles

of Geology (3 vol., 1830–33) His system was based on two

propositions: the causes of geologic change operating include all the causes that have acted from the earliest time; and these causes have always operated at the same average levels of energy These two propositions add up to

a “steady-state” theory of Earth Changes in climate have fluctuated around a mean, reflecting changes in the posi-tion of land and sea Progress through time in the organic world is likewise an illusion, the effect of an imperfect paleontological record

The main part of the

Principles was devoted less

to theory than to

proce-dures for inferring events

from rocks For Lyell’s

clear exposition of

meth-odology, his work was

highly regarded

through-out its many editions, long

after the author himself

had abandoned

antipro-gressivist views on the

development of life

Louis Agassiz and

the Ice Age

Huge boulders of granite resting upon limestone of the Jura Mountains were subjects of controversy during the 18th and early 19th centuries Saussure described these

Charles Lyell Hulton Archive/Getty

Images

in 1779 and called them erratics He concluded that they had been swept to their present positions by torrents of water Saussure’s interpretation was in accord with the tenets of diluvial geologists, who interpreted erratics and sheets of unstratified sediment (till or drift) spread over the northern parts of Europe and North America as the work of the “Deluge.”

In 1837 the Swiss zoologist and paleontologist Louis Agassiz delivered a startling address before the Helvetian Society, proposing that, during a geologically recent stage

of refrigeration, glacial ice had covered Eurasia from the North Pole to the shores of the Mediterranean and Caspian seas Wherever erratics, till, and striated pavements of rock occur, sure evidence of this recent catastrophe exists The reception accorded this address was glacial, too, and Alexander von Humboldt advised Agassiz to return to

to their source of origin and serve as indicators of the direction of glacial movement Studies making use of such indicator erratics have provided information on the general origins and flow paths of the major ice sheets and on the locations of important mineral deposits Erratics played an important part in the initial recognition of the last ice age and its extent Originally thought to be transported by gigantic floods or by ice rafting, erratics were first explained in terms of gla- cial transport by the Swiss-American naturalist and geologist J.L.R Agassiz in 1840.

his fossil fishes Instead, he began intensive field studies

and in 1840 published his Études sur les glaciers (“Studies of

Glaciers”), demonstrating that Alpine glaciers had been far more extensive in the past That same year he visited the British Isles in the company of Buckland and extended the glacial doctrine to Scotland, northern England, and Ireland In 1846 he carried his campaign to North America and there found additional evidence for an ice age

Geologic time and the Age of Earth

By mid-century the fossiliferous strata of Europe had been grouped into systems arrayed in chronological order The stratigraphic column, a composite of these systems, was pieced together from exposures in different regions by application of the principles of superposition and faunal sequence Time elapsed during the formation of a system became known as a period, and the periods were grouped into eras: the Paleozoic (Cambrian through Permian periods), Mesozoic (Triassic, Jurassic, and Cretaceous periods), and Cenozoic (Paleogene, Neogene, and Quaternary periods)

Charles Darwin’s origin of Species (1859) offered a

theo-retical explanation for the empirical principle of faunal sequence The fossils of the successive systems are differ-ent not only because parts of the stratigraphic record are missing but also because most species have lost in their struggles for survival and also because those that do sur-vive evolve into new forms over time Darwin borrowed two ideas from Lyell and the uniformitarians: that geo-logic time is virtually without limit and that a sequence of minute changes integrated over long periods of time pro-duce remarkable changes in natural entities

The evolutionists and the historical geologists were embarrassed when, beginning in 1864, William Thomson (later Baron Kelvin) attacked the steady-state theory

of Earth and placed numerical strictures on the length of geologic time Earth might function as a heat machine, but it could not also be a perpet-ual motion machine Assuming that Earth was originally molten, Thomson calculated that not less than 20 million and not more than 400 million years could have passed since Earth first became a solid body Other physicists of note put even narrower limits on Earth’s age ranging down to

15 million or 20 million years All these calculations,

how-15 million or 20 million years All these calculations, however, were based on the common assumption, not always explicitly stated, that Earth’s substance is inert and hence incapable of generating new heat Shortly before the end of the century this assumption was negated by the discovery of radioactive elements that disintegrate spontaneously and release heat to Earth in the process

Concepts of Landform Evolution

The scientific exploration of the American West following the end of the Civil War yielded much new information on the sculpture of the landscape by streams In his reports

on the Colorado River and Uinta Mountains (1875, 1876), John Wesley Powell explained how streams may come to flow across mountain ranges rather than detour around them The Green River does not follow some structural crack in its gorge across the Uinta Mountains; instead it

William Thomson, Baron Kelvin, 1869.

© Photos.com/Thinkstock

has cut its canyon as the mountain range was slowly bowed

up Given enough time, streams will erode their drainage basins to plains approaching sea level as a base

Grove Karl Gilbert’s Report on the Geology of the Henry

Mountains (1877) offered a detailed analysis of fluvial

pro-cesses According to Gilbert, all streams work toward a graded condition, a state of dynamic equilibrium that is attained when the net effect of the flowing water is nei-ther erosion of the bed nor deposition of sediment, when the landscape reflects a balance between the resistance of the rocks to erosion and the processes that are operative upon them

After 1884 William Morris Davis developed the cept of the geographical cycle, during which elevated regions pass through successive stages of dissection and denudation characterized as youthful, mature, and old Youthful landscapes have broad divides and narrow valleys With further denudation the original surface on which the streams began their work is reduced to ridgetops Finally in the stage of old age, the region is reduced to a nearly featureless plain near sea level or its inland projec-tion Uplift of the region in any stage of this evolution will activate a new cycle Davis’s views dominated geomorphic thought until well into the 20th century, when quantitative approaches resulted in the rediscovery of Gilbert’s ideas

con-Gravity, Isostasy, and Earth’s Figure

Discoveries of regional anomalies in Earth’s gravity led to the realization that high mountain ranges have underlying deficiencies in mass about equal to the apparent surface loads represented by the mountains themselves In the 18th century the French scientist Pierre Bouguer had observed that the deflections of the pendulum in Peru are much less than they should be if the